The invention relates to the area of technology of sheet metal having an aluminum-containing coating. In particular, it relates to sheet metal having an aluminum-containing coating which coating comprises an aluminum-silicon alloy. Such coated sheet metal is used, e.g. to produce heat shields for the exhaust system conduits of automobiles.
The object of a heat shield is to insulate pieces disposed behind it from a source of heat disposed in front of it. Thus, a heat shield should have the minimum possible energy absorptivity; stated otherwise, it should have maximum repellence for incident energy. Such behavior is characterized by low emissivity of the constituent material; in other words, high reflectance. Thus, desirable heat shields are comprised of materials which have satisfactory mechanical properties, have good formability for fabrication purposes, and good resistance to corrosion, and further have low emissivity.
It is known to produce heat shields from sheet metal having an aluminum-containing coating which coating comprises an aluminum-silicon alloy. An example of such coated sheet metal is low carbon steel coated on its two principal surfaces with an aluminum-silicon alloy which is applied by passing the sheet into a bath of the fused alloy. During the passage of the sheet into the coating bath comprising the aluminum-silicon alloy, a layer of an iron-aluminum-silicon alloy develops. Accordingly, the metallographic cross section of the coating reveals the following structure:
a surface layer having a composition close to that of the bath, and PA1 a subsurface layer comprising a ternary alloy, of composition Fe.sub.3 Si.sub.2 Al.sub.12. Such sheet metal with an aluminum-containing coating has a low overall emissivity, less than 0.2, and thus a high reflectivity, greater than 80%. This characteristic is maintained at temperatures up to 450.degree. C. The material is thereby of substantial engineering interest, for use for interior walls of industrial or household furnaces, heat reflectors for all manner of household appliances, or heat shields for conduits in automotive exhaust systems, but not for the relatively hotter such conduits. PA1 production of steel sheet coated on at least one of its principal surfaces by a layer of a coating in the solid state, which coating is comprised of an aluminum-based alloy comprised of aluminum and silicon, said alloy including silicon less than 11 wt. %, comprising 7-11 wt. % silicon and 87-93 wt. % aluminum; PA1 heating the coating layer to a temperature T1 which is greater than the fusion temperature T2 of said coating; PA1 maintaining the coating layer at the said temperature level T1 greater than the fusion temperature T2 of the coating, for a duration between 0 and 100 sec, preferably between 0 and 10 sec; PA1 cooling the sheet to a temperature at least equal to the limiting alloying temperature of alloying between the coating and the steel, and preferably cooling the sheet to the ambient temperature (room temperature). PA1 The temperature T1 to which the coating layer is heated is between the fusion temperature T2 of the coating layer and 650.degree. C. PA1 The temperature T1 is 10-15.degree. C. above the fusion temperature T2 of the coating layer. PA1 The rate of heating of the coating layer is in the range 20-100.degree. C. per second. PA1 The cooling of the coated sheet metal is natural cooling in the open air, or "forced" radiative cooling. PA1 The cooling of the coated sheet metal is forced-air cooling. PA1 The cooling of the sheet metal is carried out in at least two stages, as follows: PA1 The sheet steel coated on at least one of its principal surfaces by a layer of a coating in the solid state, which coating is comprised of an aluminum-based alloy of a type comprised of aluminum and silicon, said alloy comprising silicon less than 11 wt. %, is fabricated by dip-coating a steel substrate in a fused bath comprising silicon 9-10 wt. %, iron c. 3 wt. %, and the remainder aluminum, and cooling the coated substrate to a temperature less than the fusion temperature T2 of the coating.
A method of improving the properties of such materials is known wherein the material is passed through a roll stand or roll housing, known as a "skin-pass" stand, where it is cold-worked by means of smooth rolls. This process is capable of slightly reducing the emissivity of the material, but at the cost of degradation of desirable high-temperature properties.
The object of the present invention is to solve the above-described problem by devising a sheet metal having an aluminum-containing coating which coating comprises an aluminum-silicon alloy, wherewith said coated sheet metal has low emissivity and can be used as a heat shield for heat sources having temperatures above 500.degree. C., e.g. conduits of which automotive exhaust systems are formed, including the hottest such conduits.